The invention relates to the prior art requirement for monitoring the operability of structures that are subject to repetitive frictional wear; for example, brake liners of motor vehicles and skids of magnetically suspended railroad cars. With each of such structures there is a need for a rapid indication of when such structure is so worn that efficient further operability is put in question. Such a condition is evidenced upon the thickness of such structure being reduced by the wear of an operable surface of such structure to a predetermined, minimum value.
With regard to magnetically suspended railroad cars, a minimum thickness of the skids is required under temperatures in excess of 700.degree. C. Both the skids and any sensing devices employed to monitor the thickness of the skids must be so designed as to withstand extremely high temperatures.